Free Astronomy Magazine July-August 2025

27 ASTRO PUBLISHING pieces from that breakup clump to- gether to form new, loosely held-to- gether bodies. On the surfaces of these rubble-pile asteroids, we often see large boulders—some as large as ~60 meters. At 50-60 meters in size, YR4 falls into this size range, sug- gesting it could have been a boulder that once sat on the surface of a larger rubble-pile asteroid. Scientists rely on the Yarkovsky Effect to ex- plain and predict the orbits of aster- oids. There is a subtle force that happens when an asteroid absorbs sunlight and then re-emits that energy as heat. That heat, in the form of infrared light, pushes back slightly on the asteroid, slowly changing its orbit. How strongly an asteroid responds to this effect de- pends on its thermal inertia – how quickly it heats up and cools down. Small, 50-meter-sized asteroids, like YR 4 , may have low thermal inertia, which suggests they’re made of solid rock. This differs from larger, rubble-pile asteroids, which tend to JULY-AUGUST 2025 A n artist’s illustration of Earth- crossing asteroid 2024 YR 4 . [W.M. Keck Obs./Adam Makarenko]